1. Field of the Invention
The present general inventive concept relates to a method of measuring volumes of ink droplets ejected from nozzles of an inkjet head and a method of controlling the nozzles of the inkjet head using the method.
2. Description of the Related Art
In general, inkjet heads are devices that eject ink droplets onto desired positions of a recording medium to form an image. Inkjet heads are categorized into two types according to the ink ejection mechanism thereof. The first one is a thermal inkjet head that ejects ink droplets due to an expansion force of bubbles generated in ink by thermal energy. The other one is a piezoelectric inkjet head that ejects ink droplets due to pressure applied to ink due to deformation of a piezoelectric body.
Inkjet heads have recently been used in image forming and other fields. For example, inkjet heads have been used to manufacture color filters of liquid crystal displays (LCDs). Color filters have been manufactured by dyeing, pigment dispersion, printing, and electrodeposition. However, since these methods need a separate process for each color pixel, process efficiency is low and manufacturing cost is high. Thus, a method of manufacturing a color filter using inkjet printing has recently been developed to simplify a manufacturing process and reduce manufacturing costs. This method manufactures a color filter by ejecting colored ink droplets, e.g., red (R), green (G), and blue (B) ink droplets, through nozzles of an inkjet head into pixels. In addition, inkjet heads can be used to form an organic light emitting layer of an organic light emitting diode (OLED) or an organic semiconductor material of an organic thin film transistor (OTFT).
Various methods of ejecting the same amount of ink from nozzles of an inkjet head during printing have been suggested. One method is to normalize the speed of each of ink droplets ejected from nozzles. Another method is to normalize the mass of each of inkjet droplets ejected from nozzles. Another method is to normalize the volume of each of ink droplets ejected from nozzles. Also, a method of controlling the amount of ink by controlling a pulse duration or a voltage applied to nozzles has been suggested.
FIGS. 1A and 1B illustrate a conventional method of normalizing the volume of ink droplets 1a, 1b, and 1c respectively ejected from nozzles N1, N2, and N3 of an inkjet head 10 using a strobe stand. FIG. 1B is a view obtained by rotating FIG. 1A by 90 degrees. In FIGS. 1A and 1B, the ink droplets 1a, 2a, and 3a may be simultaneously ejected from all the nozzles N1, N2, and N3 of the inkjet head 10.
Referring to FIGS. 1A and 1B, using a light source 20 and a camera 30 disposed on either side of the ink droplets 1a ejected from the nozzle N1, when the light source 20 is synchronized with the nozzle N1 that operates at a specific frequency, an image of an ink dot which is formed by overlapping the ink droplets 1a is captured by the camera 30. For example, when the nozzle N1 operates at a frequency of 1 kHz and the camera 30 has a shutter speed of 1/30 sec, an image of one ink dot which is formed by overlapping approximately 30 ink droplets is captured by the camera 30. The volume of one ink droplet 1a ejected from the nozzle 1A can be calculated from the image, and thus a desired volume of each ink droplet 1a can be calculated by controlling a voltage applied to the nozzle N1 or by controlling a pulse duration. This process is repeated for the other nozzles N2 and N3. Accordingly, the same amount of ink can be ejected from all the nozzles N1, N2, and N3 of the inkjet head 10.
However, there is a limitation in applying the conventional method to printing technologies. The conventional method should simultaneously eject the ink droplets 1a, 1b, and 1c at regular time intervals from all the nozzles N1, N2, and N3 of the inkjet head 10, and can eject the same amount of ink from the nozzles N1, N2, and N3 only when pitches between print patterns formed by the nozzles N1, N2, and N3 are equal to pitches between the nozzles N1, N2, and N3, that is, when printing is performed in a state where the nozzles N1, N2, and N3 of the inkjet head 10 are arranged in a direction perpendicular to a print direction. However, it is rare for the ink droplets 1a, 1b, and 1c to be simultaneously ejected at regular time intervals from all the nozzles N1, N2, and N3 of the inkjet head 10. Also, if the pitches between the print patterns are narrower than the pitches between the nozzles N1, N2, and N3, the nozzles N1, printing is performed in a state where the nozzles N1, N2, and N3 of the inkjet head 10 are angled by a predetermined amount with respect to the print direction.
In general, the amount of ink ejected from an inkjet head varies depending on the number of nozzles that simultaneously eject ink, and cross-talk between the nozzles due to relative ejection timings of the nozzles, as well as the nature of ink and the structure of the inkjet head. Accordingly, although the same waveform is applied to the same nozzle, when the number of nozzles simultaneously ejecting ink or when an ink ejection timing is changed, different amounts of ink are ejected from the nozzles. Therefore, although the same amount of ink is expected to be ejected from the nozzles N1, N2, and N3 using the conventional method of FIGS. 1A and 1B, different amounts of ink are ejected from the nozzles N1, N2, and N3 in an actual printing process. Such a difference in amount may not be a serious problem in general image forming/printing, but may be a serious problem in more specialized fields of printing, such as color filter printing, in which the amount of ink should be precisely controlled.